The invention relates to a method for producing a thermocouple having at least two thermolegs, one formed of p-doped semiconductor material and one formed of n-doped semiconductor material, which are electrically conductively interconnected alternately on a hot and a cold side, a thermocouple that can be produced in accordance with the method, and a substrate provided for the method.
The operating principle of thermocouples is based on the thermoelectric effect. In the case of the Seebeck effect, an electric voltage is created between two points of an electric conductor or semiconductor that have a different temperature. Whereas the Seebeck effect describes the creation of a voltage, the Peltier effect occurs exclusively by the flowing of an external current. The Peltier effect occurs when two conductors or semiconductors having different electronic thermal capacities are brought into contact and electrons flow from one conductor/semiconductor into the other due to an externally applied electric current. Both thermoelectric effects occur in a thermocouple through which current is flowed.
Thermocouples preferably consist of differently doped semiconductor materials, whereby the efficiency can be considerably increased with respect to thermocouples having two different metals interconnected at one end. Conventional semiconductor materials are Bi2Te3, PbTe, SiGe, BiSb or FeSi2.
A conventional thermocouple consists of two or more small cuboids formed from p- and n-doped semiconductor material, which are interconnected alternately above and below by metal bridges. The metal bridges simultaneously form the thermal contact areas on a hot and cold side of the thermocouple and are usually arranged between two ceramic plates arranged at a distance from one another. A thermocouple is formed in each case by an n-doped and a p-doped cuboid, wherein the cuboids extend between the hot and the cold side of the thermoelectric element. The differently doped cuboids are interconnected by the metal bridges such that they produce a series connection.
If an electric current is fed to the cuboids, the connection points of the cuboids on the cold side cool according to the current intensity and current direction, whereas the connection points on the opposite hot side heat up. The applied current thus produces a temperature difference between the ceramic plates. If, however, a different temperature is applied to the opposite ceramic plates, a current flow is produced in the cuboids of the thermocouple according to the temperature difference.
The edge lengths of the cuboids in all directions are approximately 1-3 mm. The form of the cuboid is roughly approximated with a cube. The considerable thickness of the cuboids is necessary, since these serve not only to achieve the thermoelectric effect, but additionally ensure the mechanical stability of the thermoelectric element. The cuboids require a large amount of semiconductor material, which is not necessary in order to achieve the thermoelectric effect.
GB 911 828 A discloses a thermocouple, in which metal plates are soldered to the cuboids of said thermocouple both on the cold and the hot side. The metal plates arranged offset in relation to one another on the hot and cold side are each deflected by 90° along a line. The deflected regions of the metal plates on the hot and cold side are arranged parallel to one another in a mutually spaced manner. The cuboids are soldered between the deflected regions of opposite metal plates. The cuboids and parts of the plates are then cast in an electrically insulating compound that is a poor thermal conductor. The stability of the thermocouple is to be improved by the casting process. A disadvantage of the known thermocouple lies in the fact that the cuboids have to be relatively large for the placement and soldering between the deflected regions. The placement and soldering between the deflected regions also contradicts cost-effective production of the thermocouples on a mass scale.
Besides the conventional thermocouples, what are known as thin-film thermocouples are known. For example, DE 101 22 679 A1 discloses a thin-filmed thermocouple which comprises a flexible substrate material made of plastic, to which thin-film thermocouples are applied. Strip-like thermolegs arranged side by side and made of a first material and made of a second material are formed, wherein the thermolegs are electrically conductively interconnected at their ends in pairs via a coupling structure, in particular made of the second material. The thermolegs and coupling structure are applied by means of conventional deposition methods. Due to the thermolegs coupled alternately on the hot and on the cold side, a series connection of a plurality of thermocouples over a small area of approximately 8×15 mm is formed. The thickness of the thermolegs of a thin-film thermocouple lies in a range of 1-10 μm.
The thin-film technology has the disadvantage that the electrical resistance of the thermoleg is very large due to the low layer thickness, which has a disadvantageous effect on the efficiency. In order to stabilize the unstable thermolegs, these have to be applied over their entire area to the plastic substrate. The low thermal conductivity of the plastic substrate indeed in principle has a positive effect on the efficiency of the thin-film thermocouple, however it is difficult to couple and decouple heat into/from the coupling structures of the thermocouple on the hot and cold side, which are applied to the substrate made of plastic. Ultimately, the application of the thermolegs to the plastic substrate and therefore the production of the thermocouple are complex. EP 1 976 034 A discloses a further thin-film thermocouple, of which the thermolegs are arranged on a substrate having partly insulating properties and extending between two frame parts on the hot and cold side of the thermocouple.